In integrated circuits, the wiring that connects the various devices and components that make up the full chip is usually generated through design automation software. In most cases wires are laid down as straight lines that run along one of only two orthogonal directions (X and Y). Occasionally, however, a situation arises in which a wire needs to be laterally moved by a small amount (generally less than a line width) at some point along its path to avoid either overlying, underlying, or touching a particular component or device or even another wire.
The standard way of introducing such a jog into the path of a wire is illustrated in FIG. 1. Upper segment 11 and lower segment 13 are seen to be joined by diagonal segment 12. Although it would be easier to simply cut the original line at a suitable location and then but the two ends together again, after sliding either end laterally relative to the other, this would result in sharp corners (such as 31 and 32 in FIG. 3) being formed.
It is generally agreed that a structure such as the one illustrated schematically in FIG. 3 represents an undesirable way to introduce a jog into a wire. This is because sharp corners are potential high field spots at which current leakage could occur and because significant narrowing of the wire has been introduced at the jog. Objections to this way of forming a jog were valid at a time when patterns on reticles were transferred to photoresist without distortion. For this reason the diagonal design shown in FIG. 1 was widely adopted.
Originally, patterns for use in reticles were formed using special plastic sheeting (known as rubyliths) which were cut up on X-Y plotters to form the required pattern, which was then photo-reduced onto the reticle. For this technology cutting lines in a diagonal direction (as in segment 12 of FIG. 1) presented no difficulties. However, as line dimensions have grown ever smaller so that the limits of optical imaging have been approached, reticle patterns must be formed using electron beam writing. This is accomplished using a raster in a manner similar to scanning a picture onto a television screen. As is well known, sharp diagonals are notoriously difficult to display on a television screen, being always limited by the pixel size of the system. In a similar way, writing a diagonal line using standard (i.e. raster) electron beam writing requires the production of many small squares, as opposed to the ease of being able to draw objects whose edges run only in the X or Y directions. Thus, drawing diagonal lines using electron beams is very expensive. Despite this, it remains standard in the art to form jogs in the manner depicted in FIG. 1.
Another side effect of photolithography that operates close to the optical limit is the appearance of optical proximity effects (OPE). Because of interference between the edge diffraction patterns that are present in any optical image, severe distortions of the image can occur when it is transferred from reticle to resist. An example of this is seen in FIG. 2 which shows the photoresist pattern that resulted when the jogged line of FIG. 1 was imaged onto photoresist using light whose wavelength was comparable to the width of the line. Segment 11 remains relatively free of distortion (its end is outside the figure) but the end of segment 13 is seen to have been rounded as can be seen by comparing end 14 in FIG. 1 with end 24 of FIG. 2. Also significant is the fact that the relatively blunt corners associated with diagonal segment 12 in FIG. 1 have been further smoothed making for a more gentle transition from segment 11 to segment 13.
Since the line shape seen in FIG. 2 is an improvement over the de facto standard diagonal shape of FIG. 1, the prior art has tended to "leave good enough alone" and has continued to form jogged lines by using a shape of the type shown in FIG. 1 in the reticle. As we will show below, it is possible to use a shape that is much cheaper to produce by electron beam writing while still ending up with an image at the photoresist that is very similar to that shown in FIG. 2.
A routine search of the prior art was performed but no references that describe the exact structure and method of the present invention were found. Several references of interest were, however, encountered. For example, in U.S. Pat. No. 5,682,323, Pasch et al. show an OPC (optical proximity correction) system for mask design. Yamaguchi in U.S. Pat. No. 5,847,421 shows a logic-cell having an efficient OPC correction where poly width are corrected, while Garza et al. In U.S. Pat. No. 5,900,338 show an OPC method based on using design rule checkers.